The quinone of E-diethylstilbestrol (DES), a postulated metabolic intermediate derived from DES, has been synthesized by oxidation of DES in chloroform using silver oxide. The reaction product was structurally characterized by infrared, ultraviolet, nuclear magnetic resonance, and mass spectrometry. The product of oxidation of DES by hydrogen peroxide, catalyzed by horseradish peroxidase and also by rat uterine peroxidase, was shown to be identical with synthetic DES quinone based on identical u.v. spectra and on identical decomposition products. DES quinone was stable only in non-protic solvents such as chloroform. In acids, bases or protic solvents, DES quinone rearranged to Z,Z-dienestrol (beta-DIES). The half-life of DES quinone in water was approximately 40 min; in methanol it was approximately 70 min. Bacterial mutagenicity (Ames) tests did not indicate that DES quinone had mutagenic or genotoxic activity. However, DES quinone was found to bind to calf thymus DNA without any enzyme mediation at levels significantly above the binding of DES under the same conditions. Based on the binding of DES quinone to DNA, this intermediate must be considered as a possible carcinogenic metabolite of DES.